Optics for ultrafast cameras create possibilities for imaging

MIT researchers have developed novel photography optics that capture images based on the timing of reflecting light inside the optics, instead of the traditional approach that relies on the arrangement of optical components. These new principles, the researchers say, open doors to new capabilities for time- or depth-sensitive cameras, which are not possible with conventional photography optics.

Specifically, the researchers designed new optics for an ultrafast sensor called a streak camera that resolves images from ultrashort pulses of light. Streak cameras and other ultrafast cameras have been used to make a trillion-frame-per-second video, scan through closed books, and provide depth map of a 3D scene, among other applications.

Such cameras have relied on conventional optics, which have various design constraints. For example, a lens with a given focal length, measured in millimeters or centimeters, has to sit at a distance from an imaging sensor equal to or greater than that focal length to capture an image. This basically means the lenses must be very long.

In a paper published in Nature Photonics, MIT Media Lab researchers describe a technique that makes a light signal reflect back and forth off carefully positioned mirrors inside the lens system.

A fast imaging sensor captures a separate image at each reflection time. The result is a sequence of images — each corresponding to a different point in time, and to a different distance from the lens. Each image can be accessed at its specific time. The researchers have coined this technique 'time-folded optics.'

“When you have a fast sensor camera, to resolve light passing through optics, you can trade time for space,” said Barmak Heshmat, first author on the paper. “That’s the core concept of time folding. … You look at the optic at the right time, and that time is equal to looking at it in the right distance. You can then arrange optics in new ways that have capabilities that were not possible before.”

The new optics architecture includes a set of semireflective parallel mirrors that reduce, or “fold,” the focal length every time the light reflects between the mirrors. By placing the set of mirrors between the lens and sensor, the researchers condensed the distance of optics arrangement by an order of magnitude while still capturing an image of the scene.

In their study, the researchers demonstrate three uses for time-folded optics for ultrafast cameras and other depth-sensitive imaging devices. These cameras, also called “time-of-flight” cameras, measure the time that it takes for a pulse of light to reflect off a scene and return to a sensor, to estimate the depth of the 3D scene.

Co-authors on the paper are Matthew Tancik, a graduate student in the MIT Computer Science and Artificial Intelligence Laboratory; Guy Satat, a PhD student in the Camera Culture Group at the Media Lab; and Ramesh Raskar, an associate professor of media arts and sciences and director of the Camera Culture Group.

The researchers’ system consists of a component that projects a femtosecond (quadrillionth of a second) laser pulse into a scene to illuminate target objects. Traditional photography optics change the shape of the light signal as it travels through the curved glasses.

This shape change creates an image on the sensor. But, with the researchers’ optics, instead of heading right to the sensor, the signal first bounces back and forth between mirrors precisely arranged to trap and reflect light.

Each one of these reflections is called a 'round trip.' At each round trip, some light is captured by the sensor programed to image at a specific time interval — for example, a 1-nanosecond snapshot every 30 nanoseconds.

A key innovation is that each round trip of light moves the focal point — where a sensor is positioned to capture an image — closer to the lens. This allows the lens to be drastically condensed. Say a streak camera wants to capture an image with the long focal length of a traditional lens.

With time-folded optics, the first round-trip pulls the focal point about double the length of the set of mirrors closer to the lens, and each subsequent round trip brings the focal point closer and closer still. Depending on the number of round trips, a sensor can then be placed very near the lens.

By placing the sensor at a precise focal point, determined by total round trips, the camera can capture a sharp final image, as well as different stages of the light signal, each coded at a different time, as the signal changes shape to produce the image. (The first few shots will be blurry, but after several round trips the target object will come into focus.)

In their paper, the researchers demonstrate this by imaging a femtosecond light pulse through a mask engraved with 'MIT,' set 53cm away from the lens aperture.

To capture the image, the traditional 20cm focal length lens would have to sit around 32cm away from the sensor. The time-folded optics, however, pulled the image into focus after five round trips, with only a 3.1cm lens-sensor distance.

This could be useful, Heshmat said, in designing more compact telescope lenses that capture, say, ultrafast signals from space, or for designing smaller and lighter lenses for satellites to image the surface of the ground.

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